Abstract

Barramundi, Lates calcarifer, is an iconic, tropical, finfish species on which important commercial, aquaculture and recreational fisheries are based across the breadth of northern Australia. Due to this diversity in stakeholder interest, the barramundi fishery is important socio-economically for regional northern communities. As such, understanding how climate change will impact on fishery productivity and connectivity and the identification of adaptation options if the fishery is adversely affected will be of importance to the prosperity and resilience of many tropical communities.

The primary outcome of this project was to provide various stakeholders in the multifaceted barramundi fishery (including both the commercial/recreational wild fishery and aquaculture industry) with targeted scientific data and models assessing the vulnerability of this iconic species to future impacts of climate change. Access to this data will enable stakeholders to identify adaptation strategies and put in place informed planning processes ensuring the future viability of commercial and recreational activities dependent on the species.

To assess the vulnerability of barramundi to future climate impacts the project was divided up into four components, each of which addressed a separate scientific question. Firstly, a comprehensive genetic audit with high resolution DNA markers was undertaken to describe the current stock structure of wild Australian barramundi. This audit was required as there had been no previous studies that had genotyped fish from the entire Australian distribution in the one study. Additionally, other large-scale genetic audits conducted on this species were completed 25 years ago. It was possible that over this period anthropogenic impacts such as fishing and restocking, as well as the possibility of altered environment, may have disrupted historical genetic structure. Using 16 microsatellite DNA markers we were first able to demonstrate temporal stability of population genetic structure in Australian barramundi over 25 year time scales. We were then able to confirm the existance of 21 genetically distinguishable subpopulations (largely interbreeding management units) spread across the species' distribution. These management units can be grouped into six broader genetic stocks, with some stocks showing a strong signature of mixed ancestry from neighbouring stocks, while others form more genetically discrete units (potential Evolutionary Significant Units). Some evidence suggestive of natural selection on parts of the barramundi genome across the species range were detected and warrants further investigation, as it implies that some genetically divergent stocks (or subpopulations) may have become locally adapted to varying environmental conditions.

Parasites present major economic and environmental concerns for barramundi aquaculture. As well as possible impacts on barramundi itself, climate change is expected to also affect the frequency and intensity of parasite epizootics in aquaculture by enabling parasites to complete their life-cycles faster. These changes may be primarily driven by warmer water temperatures and variation in salinity. To understand the role parasites may have in future barramundi aquaculture production we first conducted a survey of parasites that can affect barramundi and performed a risk assessment to identify parasites posing high risk to the marine barramundi aquaculture industry. Experiments were also conducted on two important marine parasites, Neobenedenia and Lernanthropus latis, to predict their life-cycle response to water temperature and salinity. These studies confirmed that the life cycle of these two important species will speed with warming temperatures. Barramundi from different genetic stocks were also shown to have the same susceptibility to Neobenedenia.

The current research project also provided baseline data on aerobic metabolism and hypoxia tolerance across five sub-populations of barramundi from the different genetic stocks. Experiments did not detect any significant changes in energy metabolism or tolerance to low oxygen amongst fish from the various stocks when exposed to cool, warm and hot temperatures. Barramundi therefore appear to cope with wide ranges in environmental temperature well above that expected to occur under climate change, as well as to possess a high-degree of tolerance to temperature-induced hypoxic episodes.

Finally, models were developed incorporating both biological characteristics and environmental projections from 18 "business as usual" climate scenarios to predict changes in the future species' distribution and catch per unit effort (CPUE) of the wild fishery, and productivity and geographical range suitable for aquaculture. All projected climate scenarios were consistant and predicted an expansion in the distribution of barramundi in a southward fashion towards the year 2085, as well as thermal regimes suitable for barramundi pond aquaculture. Habitat for barramundi in northern Australia was predicted to expand inland, particularly in the Northern Territory and Gulf of Carpentaria. CPUE is predicted to increase, indicating that fisheries in most regions may become more productive. Aquaculture productivity linked to barramundi growth models was also predicted to increase with warming conditions, primarily due to higher minimum winter temperatures and an increase in temperatures around the optimum for growth in the species.

Overall, the results from this project highlight a degree of flexibility and resilience of barramundi to cope with temperatures and environmental regimes predicted by future climate scenarios. Physiological tests demonstrate that the species performs robustly under a wide range of thermal conditions even when challenged with temperature rises expected to occur by 2085 (~3.6 °C), whilst models predict increased productivity and expansion of the fishery. Adaptation strategies and future planning processes need to consider and account for the real possibility of a southward expansion of thermal habitat suitable for the wild fishery and aquaculture production.​